1. Field of the Invention
The present invention relates to a copier, printer, facsimile apparatus or similar electrophotographic image forming apparatus and more particularly to a cleaning device included in an image forming apparatus for removing various residues, including toner contained in a developer, paper dust and additives contained in toner and a sheet, left on a photoconductive element or similar image carrier after image transfer.
2. Description of the Background Art
Generally, an electrophotographic image forming apparatus includes a photoconductive element, which is a specific form of an image carrier. A charging step and an image writing step are sequentially executed with the photoconductive element to form a latent image thereon. Subsequently, the latent image is developed by a developing step using toner to become a toner image. The toner image is transferred to a sheet or recording medium by an image transferring step and then fixed on the sheet by a fixing step.
In the image transferring step, the toner image is not entirely transferred from the photoconductive element to a sheet, but is partly left on the photoconductive element. The image transferring step is therefore followed by a cleaning step for removing the toner and other residues, i.e., paper dust, rosin, Mg, Al, K, Na and other additives contained in the sheet from the photoconductive element together with residual charges, thereby preparing the photoconductive element for the next image formation. Also left on the photoconductive element after image transfer are additives contained in the toner for implementing chargeability, fixability, fluidity and other characteristics required of the toner. Further, discharge products derived from, e.g., corona discharge are removed from the photoconductive element by the cleaning step.
A cleaning device for effecting the cleaning step may include a blade contacting the photoconductive element and a rotatable brush having looped tips held in rubbing contact with the photoconductive element, as taught in, e.g., Japanese Utility Model Laid-Open Publication Nos. 2-149969 and 2-140563 and Japanese Utility Model Publication No. 7-33260. The blade and brush are so configured and arranged to damage the photoconductive element little when removing the residues.
In the cleaning device described above, the brush needs certain pressure for scraping off the residues from the photoconductive element. It has been reported that the brush can efficiently remove the residues when contacting the photoconductive drum with pressure of 50 g/cm2 or above. This pressure refers to one that causes the tips of the brush bite into the surface of the photoconductive drum by a preselected amount.
When the brush is constantly held in contact with the photoconductive element, the following problems arise, depending on the amount of residues left on the photoconductive element. The residues removed by the brush and blade are conveyed thereby and then collected. However, when the amount of image formation effected on the photoconductive element, i.e., when an image with a small area ratio is repeatedly formed on the photoconductive element, the amount of residues to reach the cleaning device decreases little by little. Consequently, when the residues to be removed are absent or almost absent on the brush or cleaning member, the brush directly contacts the photoconductive element and charges it by triboelectrification, thereby making a charge distribution, including background potential, irregular.
The looped tips of the brush have greater bending strength than fur-like tips and therefore tend to rub the photoconductive element with higher pressure, aggravating triboelectrification on the photoconductive element. The resulting charge is apt to work as restraint on the residues left on the photoconductive element, obstructing the removal of the residues. As a result, heavy load acts on the blade, which is used in combination with the brush, and accelerates the deterioration of the blade, thereby rendering cleaning defective. In addition, maintenance cost increases due to frequent replacement of the cleaning member.
The blade of the cleaning device may be configured to remove the residues by grinding the surface of the photoconductive element, as also proposed in the past. In this connection, a method of feeding an abrasive for grinding has also been proposed. However, when the surface of the photoconductive element is ground, it is likely that the abrasive and part of a photoconductive layer shaved off are introduced in a developer. This not only lowers image quality, bur varies chargeability due to the variation of the thickness of the photoconductive layer and scratches the surface of the photoconductive layer.
In light of the above, the blade or similar stationary cleaning member may be replaced with a rubber roller or similar elastic member movable by following the movement of the surface of the photoconductive element. This, however, brings about another problem that when the photoconductive element is in a halt, the residues between the photoconductive element and the rubber roller remain pressed against the photoconductive element and are therefore positively deposited on the photoconductive element.
The amount by which the surface of the photoconductive element is ground may be reduced if frictional resistance between the above surface and the cleaning member is reduced or if the hardness of the surface is increased. However, when frictional resistance is reduced, the cleaning member is apt to fail to remove discharge products different from the residues described above. Typical of discharge products are ozone, NOx (nitrogen oxides) and a nitric compound (ammonium nitrate) derived from a nitrogen oxide thereof. Such discharge products are produced not only when discharge is effected in a space, but also when a charge roller is held in contact with the photoconductive element. The nitric compound, which is moisture-absorptive, has high resistance in a low humidity environment, but has low resistance in a high humidity environment due to moisture absorption. When the nitric compound, among others, accumulates on the photoconductive element little by little and extends over both of an image portion and background, the charge of the background is apt to migrate toward the image portion, increasing the potential of the image portion. This obstructs the deposition of toner on the image portion and is likely to bring about the local omission of an image.
The higher the surface hardness of the photoconductive element, the higher the resistance to wear and therefore the easier the removal of the residues and discharge products by the blade. An amorphous silicon is one of substances that effectively provide the photoconductive element with a hard surface.
In Laid-Open Publication Nos. 2-140563 and 2-149969 mentioned earlier, the brush and blade are respectively positioned at the upstream side and downstream side in the direction of movement of the photoconductive element in order to reduce the amount of grinding of the photoconductive element. However, the brush used to clean the photoconductive element brings about the problems stated earlier, depending on the amount of residues collected from the photoconductive element.